Physics 30
Unit Four: Nuclear Physics
Text: Merrill Physics: Principles and Problems
Radioactive Decay and Half-life
1) Radioactive Decay
 The changing of one element into another is called transmutation o Alpha and beta decay are two natural methods of transmutation
 Artificial transmutation is changing the number of protons of a nucleus artificially
 Alpha (ά) decay involves the emission of an alpha particle
 Beta (β) decay involves the emission of a beta particle
Alpha Decay
 A PARENT nucleus (which is unstable) decays into a smaller DAUGHTER nucleus o When this happens there is a decrease in atomic mass and atomic number)
 The nucleus of the parent loses an alpha particle
 Note: Atomic number and atomic mass are CONSERVED in this reaction
 In alpha decay, the mass number decreases by 4 while the atomic number decreases by 2
Beta Decay
 In beta decay, a neutron from the nucleus of the parent disappears and becomes a proton and an electron
 So an example appears as follows:
 Note: neutron in the nucleus disappears and an additional proton and electron appear – therefore the mass number is conserved o The electron is not a valence electron – it originates from the nucleus
Gamma Decay
 When the nucleus undergoes alpha or beta decay, it may contain too much left over energy
 This excess energy gets emitted in the form of gamma rays
 Gamma rays do not change the mass or atomic numbers
Radioactive Decay Series
 Sometimes the daughter nucleus that is formed from the decay of a radioactive parent nucleus is ALSO radioactive and will decay. This results in a decay series. o This decay series can involve alpha decay, beta decay, or both
 Decay series end when a stable isotope is created
 Radioactive isotopes are called radioisotopes
 Do determine what is happening during these decays, note the following: o If the mass number changes by 4, it is an alpha decay o If there in no change in mass number, it is a beta decay
 There are three common decay series o Uranium, Actinium, and Thorium decay series

2) Half-Life
Decay Constant
 All radioactive elements decay at different rates (each element decays differently)
 The activity of an element is directly proportional to the amount of radioactive material in contains
 We can express this mathematically:
A N
Where: A = activity (in Bq)
 This can be re-expressed as:
N = number of radioactive nuclei
A = λ N
Where λ is the decay constant (this measures how fast a nuclei gives off radioactivity)
Half-Life
 Radioactivity of radioactive materials decreases with time
 A half-life is the amount of time taken for half of a given number of radioactive atoms of a radioactive element to decay
 The half-life period of an element tells us whether the decay is relatively slow or fast
 The half-life cannot be affected by temperature, chemical, or physical changes
Artificial radioactivity
 The Curies (Marie and Pierre) found that a non-radioactive element could be made radioactive by bombarding the nucleus with neutrons or deutrons (a nuclei of deuterium – 1 proton + 1 neutron)
 For example:
 Cobalt-60 produces beta and gamma rays o These gamma rays are used to treat cancer
Producing Radioisotopes of Iodine
 A Neutron Howitzer is an apparatus that supplies a neutron beam
 The source of the neutron beam is a capsule of Americium-241 & Beryllium-9
 Americium-241 undergoes alpha decay which causes the Beryllium-9 to produce neutrons
 These neutrons then interact with Iodine-127 to produce radioactive Iodine-128